The present invention relates to a micromechanical device, in particular an acceleration sensor, having a seismic mass which is resiliently supported on a substrate via a first flexural spring device and which can be deflected in at least one direction by an acceleration, the deflection being able to be limited by a stop device.
Although applicable to any micromechanical devices and structures, in particular sensors and actuators, the present invention and its underlying problem are explained with regard to a micromechanical acceleration sensor which can be manufactured in the technology of silicon surface micromechanics.
Acceleration sensors, and particularly micromechanical acceleration sensors in the technology of surface micromechanics or bulk micromechanics are gaining larger and larger market segments in automotive equipment applications, increasingly replacing the piezoelectric acceleration sensors customary in known methods heretofore.
The known micromechanical acceleration sensors usually work in such a way that the resiliently supported seismic mass device, which can be deflected in at least one direction by an external acceleration, produces a change in capacitance of a differential capacitor device connected thereto in response to a deflection, the change in capacitance being a measure for the acceleration.
Known, in particular, are acceleration sensors in which the deflection of the seismic mass can be limited by a fixed limit stop which is accommodated, for example, in an opening of the seismic mass.
In the known acceleration sensors, it has turned out to be a disadvantage that, subsequent to overload accelerations, the seismic mass, as the central electrode, can adhere to such fixed limit stops due to adhesive forces and/or because of electrostatic forces resulting from charges since the restoring force of the springs is too low.
On the other hand, increasing the restoring force of the springs would negatively influence the measuring sensitivity.
The micromechanical device according to the present invention has the advantage that the resiliently supported mechanical limit stops effectively prevent the seismic mass from adhering to the limit stops.
The basic idea of the present invention lies in that the stop device has at least one limit stop which is resiliently supported on the substrate via a second flexural spring device. The second flexural spring device expediently has a greater flexural strength (stiffness) than the first flexural spring device, i.e., it is a xe2x80x9chardxe2x80x9d spring.
According to a preferred embodiment, the stop device has at least one limit stop fixedly supported on the substrate. Thus, a combination of one resilient limit stop and one fixed limit stop is obtained.
According to a further preferred refinement, the limit stop which is resiliently supported on the substrate is joined to the limit stop which is fixedly supported on the substrate via the second flexural spring device. Because of this, no additional substrate anchoring is necessary for the resiliently supported limit stop.
According to another preferred embodiment, a stop device is provided in an opening of the seismic mass. This saves space and enables the stop device to be accommodated with protection.
According to a further preferred embodiment, the stop device is designed in such a manner that the resiliently supported limit stop enters into action in response to a first deflection magnitude, and the fixedly supported limit stop enters into action in response to a second deflection magnitude, the first deflection magnitude being smaller than the second deflection magnitude. Thus, the seismic mass is first decelerated prior to getting to the fixed limit stop where it is stopped abruptly. In this context, the prestress of the elastic limit stops should be dimensioned such that adhesion is prevented.
According to a further preferred refinement, the stop device has one or a plurality of projections which the stopping effect is concentrated on. This further reduces the risk of adhesion since the contact area is limited to few points.
According to another preferred embodiment, the clearance of the projections in the region of the resiliently supported limit stop(s) is smaller than in the region of the fixedly supported limit stop(s).
According to a further preferred embodiment, the stop device has a limit stop which is fixedly supported on the substrate in an opening in the seismic mass, the second flexural spring device extending from the limit stop into an opening of a movable electrode laterally attached thereto, the resiliently supported limit stop being provided essentially at the end of the second flexural spring device. This has the advantage that the second flexural spring device can be have a longer design, and that its flexural strength can therefore be adjusted more accurately.